A large number of devices for circulating a fluid in a circuit by controlling the flow rate with more or less precision are already known.
When relatively precise adjustment is required two solutions may be envisaged, namely adjusting the speed or power of the active member (the pump) by providing it with a converter or a drive which is continuously adjustable within the required ranges, or adding a flow-adjusting means with a movable element such as a valve or the like to an active element with limited or crude adjustment capacity (such as a pump operating at constant speed).
The first solution is relatively expensive owing to the type of motor and electronic control means needed for its use and the second solution, although less costly, often fails to provide the required precision in many applications, because the adjusting means used do not take the actual position of the movable element into account and/or do not allow for the physical changes affecting the mechanical parts of the devices.
In the latter case wear on the packing of the movable element and/or the contact surface of that element will lead to a shift in the fully-closed position of the adjusting device which will not be taken into account.